The present disclosure relates to a method and apparatus for removing salts from a liquid solution.
Many parts of the world, and a substantial portion of the world's population, suffer from a shortage of water. Although the oceans cover about two-thirds of the earth's surface, this water is unsuitable for most agricultural uses and for domestic consumption because of its high salt concentration. Ocean water typically contains about 35 parts per thousand (ppt) of dissolved salts, including sodium chloride.
Various methods have been developed for removing salts from seawater. Conventional methods include ion exchange, reverse osmosis, electro dialysis, electro deposition, and filtering. In reverse osmosis systems, water is forced through a membrane, which acts as a filter for separating ions (and other impurities) from the water. Reverse osmosis systems suffer several disadvantages, including substantial energy consumption and membrane degradation.
Ion exchange systems, such as residential water softeners and boiler water treatment systems for nuclear and fossil fuel power plants, produce water having a relatively high concentration of sodium chloride. Thus, an additional desalination step, such as reverse osmosis, is needed in order to make the water suitable for human consumption.
Other methods have been proposed to address the shortcomings of the conventional separation processes. In a capacitive deionization (CDI) process, an electrical potential difference is applied over two porous carbon electrodes. Negatively charged ions (anions) are attracted to the electrode having the lower electrical potential (anode), while positively charged anions (cations) are attracted to the electrode having the higher electrical potential (cathode).
A CDI system cycles through two phases: an adsorption phase in which water is desalinated and a desorption phase which the electrodes are regenerated. During the adsorption phase, a potential difference is applied over the two electrodes, and ions are adsorbed from the water. When the electrodes become saturated with ions, regeneration is necessary. During regeneration, the potential difference between the electrodes is reversed or reduced to zero. The ions leave the electrode pores and can be flushed out of the CDI cell.
Despite the emergence of the CDI methods, limitations in the technology for separating salts from liquid solutions still exist.